ceph: don't use -ESTALE as special return code in try_get_cap_refs

In some cases, we may want to return -ESTALE if it ends up that we're
dealing with an inode that no longer exists. Switch to using -EUCLEAN as
the "special" error return.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>